The invention relates to an electronic system for controlling the operation of an automatic, variable speed transmission of a vehicle, and in particular, to an electronic shift control system which automatically determines a speed stage or gear ration to be changed in accordance with the throttle opening and the vehicle speed and in which the switching to the desired speed stage or gear ratio is automatically achieved by a selective energization of solenoid valves contained in an oil pressure circuit.
In a vehicle having an automatic, variable speed transmission mounted thereon, a speed stage or gear ratio is determined on the basis of a throttle opening and a vehicle speed, and when a throttle opening and a vehicle speed reaches a preset region, a shift or speed change takes place automatically. However, because of a limited number of reference data or patterns of a throttle opening vs. vehicle speed which can be relied upon to determine the speed change, there have been certain inconveniences experienced in the prior art arrangement. By way of example, when moving from a flat road to an up slope, a driver will recognize a reduction in the vehicle speed, and accordingly depresses an accelerator pedal in order to maintain the vehicle speed. This results in a down shift to a lower speed stage, increasing the driving power. Consequently, the vehicle speed increases, but when the intended vehicle speed is exceeded, the accelerator pedal is partially released. Thereupon, there results an up shift to a higher speed stage or gear ratio. An operation of the accelerator pedal in this manner results in a so-called hunting, that is, a frequent repetition of speed changes. When running on a down slope, a lesser magnitude of the driving power is sufficient, so that the accelerator pedal is generally partially released. This results in an up shift to a higher speed stage or gear ratio, causing an unintended increase in the vehicle speed since an engine brake operation cannot be activated, requiring a frequent use of a brake.